Electric fuse with improved casing

ABSTRACT

An electric fuse structure comprising a generally tubular electrically insulating or dielectric casing in which a fusible element is disposed and which is substantially filled with a finely divided or pulverulent arc extinguishing material, such as quartz sand. The casing is coated on its inner surface with a thin layer of ceramic electrically insulating material. The ceramic material hardens in place to form a relatively thin insulating liner.

United States Patent [1 1 Blewitt et al.

ELECTRIC FUSE WITH IMPROVED CASING lnventorsf' Donald D. Blewitt, Pittsburgh; Frank L. Cameron, Irwin, both of Pa.

Assignee: Westinghouse Electric Corporation,

Pittsburgh, Pa.

Filed: Feb. 16, 1971 Appl. No.: 115,196

US. Cl 337/246, 337/231, 337/280 Int. Cl. Hlh 85/14 Field of Search 337/228, 246, 247, 270, 337/279, 282, 231, 82, 280; 106/46; 117/95, 200; 174/5051, 52 S; 338/262, 266

[ 6] References Cited UNITED STATES PATENTS Weston Kozacka Hart et al 337/160 337/246 UX Jacks et al. 337/246 X Rigterink 338/266 X Casy 337/82 Apr. 2, 1974 FOREIGN PATENTS OR APPLICATIONS 20,483 7/1903 Great Britain 337/231 764,006 8/1968 Germany ..337/228 OTHER PUBLICATIONS Handbook of Chemistry & Physics, 43rd Edition, page 3359, 1962.

Primary Examiner-Bemard ArGilheany Assistant Examiner-F. E. Bell Attorney, Agent, or FirmW. A. Elchik [5 7] ABSTRACT An electric fuse structure comprising a generally tubular electrically insulating or dielectric casing in which a fusible element is disposed and which is substantially filled with a finely divided or pulverulent arc extinguishing material, such as quartz sand. The casing is coated on its inner surface with a thin layer of ceramic electrically insulating material. The ceramic material hardens in place to form a relatively thin insulating liner.

6 Claims, 3 Drawing Figures PATENTEDAPR 2 I974 YAI/l/ g/ll/l/ INVENTORS Donald D. Blewifl l.

WITNESS-ES Frank L. Cameron QQWW m ML ATTORNEY 1 ELECTRIC FUSE WITH IMPROVED CASING BACKGROUND OF THE INVENTION tubular electrically insulating casings or housings which surround an associated fuse element and granular filler material, and which may char or burn during element operation when the fuse elements fuses or melts and one or more arcs results. The dielectric material of the casing usually chars because it is made of synthetic resin,such as melamine resin material, which may be reinforced with glass laminations. These materials are used because they have dielectric and structural characteristics which are suitable to normally withstand relatively high voltages, and to withstand certain pressures which may result during the operation of such fuses. However, the previously mentioned charring or buring of the casing may have a tendency to weaken this dielectric characteristic.

The electrically insulating casing is usually disposed intermediate the ends of a fuse structure which includes a pair of electrically conducting ferrules, at-

tached to the opposite ends of the electrically insulating casing to provide electrical terminations for the associated fuse element which is connected therebetween and to provide a closure means at each end of the easing. The fuse structure may be a high voltage type'so that when the-fuse interrupts a circuit due to overload current, each end ferrule will be at a different voltage or potential. In a situation such as this, the casing of the It has been found that the burned or charred dielectric material of the casing may not retain a sufficient dielectric characteristic or strength to withstand longitudinal voltage breakdown between the associated end ferrules of a fuse, of the type described, because the fulgurite or amalgamation of fused'granular filler which usually forms when a-fuse element melts and burns back may act as a temporary conductor of electrical current between the remaining ends of the fuse element and the charred casing, whereby a current carrying path may result which extends longitudinally'across the charred or burned material of the casing and the combination of the fulgurite and the remaining unburned ends of the fuse element so that a voltage breakdown may occur between the associated end ferrules even though the dielectric material of the casing is sufficient considered alone or by itself to withstand a voltage breakdown across its axial length. It has been suggested that to prevent this situation, a ceramic or similar mate rial be used to form the fuse casing or tubular structure in place of the synthetic resin material because it resists burning or charring. However, ceramic tubular sections are difficult to form or to machine and are expensive and brittle or breakable. A problem therefore results in providing a fuse casing where it is necessary to choose between the more flexible, less expensive, synthetic resin type of insulating casings which are more easily burned and which may therefore break down when subjected to high voltage and the more brittle, expensive, ceramic casings which are difficult to machine, but which can withstand high voltage, and which will not char or burn.

It would be advantageous to provide a fuse structure including a tubular structure or casing of the type previously described which combines the desirable characteristies of both types of materials available.

SUMMARY OF THE INVENTION In accordance with the invention, a fuse structure includes a synthetic resin, electrically insulating, hollow, cylindrical or tubular casing in one embodiment to enclose the intermediate portion of the fuse structure, a pair of electrically conducting end ferrules which are also electrically connected to an internally disposed fuse element formed from a fusible material, such as a silver ribbon or wire. The remaining space in the casing may be substantially filled with a granular or pulverulent arc quenching material, such as quartz sand, to help absorb the thermal energy during circuit interruption and to assist in quenching any arcs which may result from the fusing or melting of the fuse element. The inner surface of the previously mentioned casing may be coated with a relatively thin layer of fluid ceramic paste such as the type known under the trade name SAUEREISEN which subsequently hardens to form a relatively thin shell or liner of ceramic material which is bonded to the inner surface of the outer casing. The ceramic liner has the property of reducing the transmission of heat which is generated within the fuse structure during its operation, thus reducing or eliminating any burning or charring of the outer casing. In addition, the ceramic liner has the more important property of acting as an electrical insulator between a current carrying fuse element or a portion of a fuse structure which has formed a fulgun'te path for temporary electrical current conduction and the associated casing thus preventing current carrying paths between the previously mentioned end ferrules or between an internal fuse element proximate external electrical conductors.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the invention, reference may be had to the preferred embodiment, exemplary of the invention, shown in the accompanying drawings, in which:

FIG. 1 shows a side elevation in section of a portion of a fuse assembly or structure embodying the invention;

FIG. 2 shows a similar view in side elevation of a portion of a fuse assembly embodying the invention which is adapted to be used as part of a cartridge type fuse and where the fuse element has fused or melted; and

FIG. 3 shows an orthogonal view, partially cut-away, of a cartridge type fuse embodying the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings, and FIG. I in particu lar, a portion of an electrical fuse assembly or fuse structure 10 is shown. Fuse structure 10 comprises a generally tubular outer shell or casing 12 which may be constructed of melamine resin impregnated glass laminated cloth material and which serves as an enclosing 'or housing means and an electrical insulator for fuse element 14. The balance of the space inside the casing 12 may be substantially filled with a granular or pulverulent arc quenching material 16, such as quartz sand. Tubular or hollow cylindrical, dielectric or insulating member 12 has disposed along its inner surface 19 a relatively thin layer 20 of ceramic or similar paste which may be inorganic such as sold under the trade name SAUEREISEN Cement or Insalute Cement which is a suspension of ceramic powders in a sodium silicate solution. The electrically insulating, ceramic paste 20 hardens in place and acts as an electrically and thermal insulating medium or barrier between current conducting or carrying fuse element 14 and any adjacent body which is disposed external to generally tubularcasing l2.

Referring now toFIG. 2, a portion of a fuse assembly is shown which is similar to the fuse assembly 10 and which forms part of a cartridge fuse structure which includes an end section 22 which is disposed to the right of fuse assembly 10. End section 22 includes electrically conducting terminal 24 interposed electrically between fuse element 14R and end ferrule or cap 26.'As can be seen in FIG. 2, fuse assembly 10' illustrates the operation of a fuse'structure in which the remaining portions of fuse element as indicated at'l4L and 14R result at the left and right ends of fuse assembly 10' respectively. A fulgurite or amalgamation of fused or vitrified sand particles 28 is shown disposed between fuse element portions 14L and 14R. The fulgurite 28 may be electrically conductive temporarily between fuse element portion 14L and outer casing 12'. It will be noted that the heat from the operation or fusion of the fuse assembly 10' may cause outer casing 12' to char as is shown at two different locations as indicated at 30 and 32 respectively, which may result from prolonged heating of the casing 12' which occurs when the'fuse element 14 is subjected to relatively low overload currents for which the melting interval is a matter'of minutes or hours. Location 30 represents a relatively small charring of the fuse material 12 whereas location 32 represents a relatively larger, charred portion of casing 12. As can be seen, a potential difference Vl results between blown fuse element portion 14L and external conductor 35 such that in the absence of the ceramic liner a possible voltage I breakdown path 34 might result. However a ceramic or similar liner will insulate against a voltage breakdown. The potential breakdown path will be from the blown fuse element portion 14L through fulgurite material 28, through ceramic coating 20', through the charred material to an electrically conducting body 35. Also, a second voltage or potential V2 may exist which might cause a breakdown in the absence of the ceramic liner 20 between fuse elementportion 14L and ferrule 26. However a ceramic or similar liner 20 will insulate against such a voltage breakdown. The possible voltage breakdown paths may result through either of two paths indicated at 36 and 38. Path 36 extends from fuse element portion 14L, fulgurite 28, ceramic liner 20", charred dielectric material 30, the remaining portion of the casing 12' which, because of its relatively shortened distance, may not be able to withstand the high voltage impressed upon it, and finally, to ferrule 26 through end plate or terminal 24. An alternate path 38 exists between the burned away fuse element portion 14L, fulgurite portion 28A, ceramic liner 20, the charred portion 32 of the casing 12, back through ceramic liner 20, through a second portion of fulgurite 28B, through the remaining part of fulgurite 28 to the right-hand fuse element portion 14R, to end plate or terminal 24 and finally to end ferrule or end cap 26.

It is desirable in a blown fuse such as indicated at 10', to prevent voltage breakdown, arcing or short-circuit current flow due to the potential differences V1 or V2 along the possible breakdown paths 34, 36 or 38. The dielectric and thermal characteristics of the hardened ceramic liner 20' prevents a high voltage breakdown along any of the previously mentioned paths 34,36 or 38.

It will be noted that ceramic liner 20' is securely bonded to the outer casing 12 by spraying or painting it on the inner surface 19' of casing 12' so as to form an integral structure. It is contemplated that the thickness of the hard ceramic liner or insulator 20' will be about 0.02 inch to 0.05 inch.

Referring now to FIG. 3, a cutaway view of a cartridge fuse 10" is shown embodying a hardened electrically insulating ceramic liner 20". End ferrule 26" is shown as well as outer tube or casing 12". Contained within the inner periphery of casing 12'' is a ribbon of electrically conducting material 14" which may be silver and which may contain a plurality of holes 15" for a current limiting operation of the fuse. Granulated or pulverulent arc quenching material, such as quartz sand or powder 16" may substantially fill the balance of-the space within the tube or casing 12". It will be noted that a mass of melted granular material or filler forming fulgurite, such as the fulgurite 28 shown in FIG. 2, is not shown in FIG. 3 since fuse structure 10" is illustrated prior to a circuit interrupting operation.

It is to be understood that although only fuse structures 10, 10' and 10" are illustrated, that the teachings of the invention are readily adapted to cartridge type fuse structures, .the teachings of the invention may be applied to other than cartridge fuses or to current limiting fuses. It is to benoted that the teachings of the invention may be appliedto any type of fuse structure,

electrically insulating refractory material. It is also to be understood that although an arc quenching material, such as quartz sand or silica material 16 is shown in the fuse structures of FIGS. 1, 2 and 3, it isnot essential that this material be provided in a particular application of the invention. It is also understood that the charring as indicated at 30 and 32 in FIG. 2 may result from overload of current flow in a fuse prior to melting of the fuse element, as a result of operative explosion in a blown fuse or merely as the result of a less than catastrophic generation of heat in a blown fuse. In addition, dielectric deterioration of the tubular means 12 as indicated by charred sections 30 and 32, need not extend radially inwardly to meet ceramic liner 20' to require the use of the ceramic liner 20' to prevent voltage breakdown.

The apparatus embodying the teachings of this inven- 7 tion, has a number of advantages. For example, a high voltage cartridge fuse may be constructed using easily machined, relatively inexpensive. and flexible synthetic resin material or glass laminated reinforced organic material as an electrically insulating outer casing while eliminating the danger of voltage breakdown once the fuse has been blown, because of the integrally bonded ceramic insulating liner provided along the inner periphery of the outer casing. Another advantage lies in the fact that the electrically insulating, ceramic liner or similar inorganic refractory material also acts to retard the flow of heat in a radial direction thus tending to prevent the outer casing from charring initially.

We claim as our invention:

1. A fuse structure comprising, a generally tubular, electrically insulating outer casing, terminal means disposed adjacent to each of the opposite ends of said casing, a fuse element disposed inside said casing and connected between said terminal means, and an electrically insulating, relatively thin, in comparison with said outer casing, refractory liner bonded to and contacting only the inner surface of the outer casing and extending axially to substantially the ends of said outer casing and being disposed radially around said fuse'element but spaced therefrom to enclose said fuse element radially to thereby assist in electrically insulating said outer casing from. said fuse element and said terminal means from one another during at least certain operating conditions of said fuse structure.

2. The combination as claimed in claim 1, wherein said refractory liner comprises a ceramic paste applied to the inner surface of said outer casing which hardens in place to bond said liner to said outer casing.

3. The combination as claimed in claim 2 wherein said casing is formed from a melamine impregnated glassreinforced material, a quantity of granular, arc quenching material is disposed in said casing to embed said fuse element, said liner being disposed to assist in electrically insulating said casing from said fuse element when a voltage difference results between the blown ends of said fuse element after said fuse element melts.

4. A cartridge fuse comprising a pair of spaced end ferrules, a dielectric generally tubular outer member extending axially between said end ferrules, a fuse element disposed within said tubular outer member, a refractory electrically insulating liner bonded to and contacting only the inner surface of said outer tubular member, said fuse element being connected between said end ferrules, a quantity of granular arc quenching filler material disposed within said tubular member and generally surrounding said fuse element, said liner comprising a relatively thin layer of ceramic paste, when compared with said outer member, applied to the inner surface of said tubular member and being disposed radially around said fuse element but spaced therefrom to enclose said fuse element radially so as to subsequently harden in place and provide electrical insulation which prevents both a voltage breakdown in a radial direction with respect to said fuse element and in an axial .direction between said end ferrules when said fuse element melts.

5. A cartridge fuse as claimed in claim 4, wherein said ceramic comprises an inorganic ceramic paste and said tubular member comprises a generally hollow cylindrical melamine resin impregnated reinforced glass material and said granular filler comprises quartz sand.

6. A cartridge fuse as claimed in claim 5 wherein said inorganic ceramic paste comprises a suspension of ceramic powders in sodium silicate solution. 

1. A fuse structure comprising, a generally tubular, electrically insulating outer casing, terminal means disposed adjacent to each of the opposite ends of said casing, a fuse element disposed inside said casiNg and connected between said terminal means, and an electrically insulating, relatively thin, in comparison with said outer casing, refractory liner bonded to and contacting only the inner surface of the outer casing and extending axially to substantially the ends of said outer casing and being disposed radially around said fuse element but spaced therefrom to enclose said fuse element radially to thereby assist in electrically insulating said outer casing from said fuse element and said terminal means from one another during at least certain operating conditions of said fuse structure.
 2. The combination as claimed in claim 1, wherein said refractory liner comprises a ceramic paste applied to the inner surface of said outer casing which hardens in place to bond said liner to said outer casing.
 3. The combination as claimed in claim 2 wherein said casing is formed from a melamine impregnated glass reinforced material, a quantity of granular, arc quenching material is disposed in said casing to embed said fuse element, said liner being disposed to assist in electrically insulating said casing from said fuse element when a voltage difference results between the blown ends of said fuse element after said fuse element melts.
 4. A cartridge fuse comprising a pair of spaced end ferrules, a dielectric generally tubular outer member extending axially between said end ferrules, a fuse element disposed within said tubular outer member, a refractory electrically insulating liner bonded to and contacting only the inner surface of said outer tubular member, said fuse element being connected between said end ferrules, a quantity of granular arc quenching filler material disposed within said tubular member and generally surrounding said fuse element, said liner comprising a relatively thin layer of ceramic paste, when compared with said outer member, applied to the inner surface of said tubular member and being disposed radially around said fuse element but spaced therefrom to enclose said fuse element radially so as to subsequently harden in place and provide electrical insulation which prevents both a voltage breakdown in a radial direction with respect to said fuse element and in an axial direction between said end ferrules when said fuse element melts.
 5. A cartridge fuse as claimed in claim 4, wherein said ceramic comprises an inorganic ceramic paste and said tubular member comprises a generally hollow cylindrical melamine resin impregnated reinforced glass material and said granular filler comprises quartz sand.
 6. A cartridge fuse as claimed in claim 5 wherein said inorganic ceramic paste comprises a suspension of ceramic powders in sodium silicate solution. 